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Comparative studies of natural groups of organisms
blood. Moreover, they have closed Chondrichthyes include sharks, skates,
circulatory system. and rays.
(f) They are triploblastic, coelomate
animals with bilaterally symmetrical Distinctive features of class
body. Chondrichthyes
(g) Most vertebrates have two pairs of Members of the class Chondrichthyes
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pentadactyl limbs or fins formed have the following unique features,
from more than one body segment. In which distinguish them from other fish
higher chordates, they are attached to types.
the rest of the skeleton by pelvic and (a) They possess placoid scales (tooth-
pectoral girdles. like) covering their skin.
(h) Some members (cartilagenous fish, (b) They have cartilaginous endoskeleton.
bony fish, amphibians and reptiles) (c) Most have five to seven pairs of
are poikilothermic organisms, in that visceral clefts as separate gill slits for
their body temperature depends on gas exchange,with no opercula or gill
environmental temperature changes, covers.
while others (birds and mammals) (d) They have a mouth located located
ventrally.
are homeothermic organisms, as their (e) Most have heterocercal tail fins
body temperature remains constant (asymmetric in shape), which prevent
regardless of the changes in external them from sinking as they lack swim
environmental temperature. bladder for buoyancy.
(f) They have paired fleshy pectoral and
Classes of phylum Chordata pelvic fins for swimming.
There are several classes of phylum (g) Males have claspers for internal
chordata, however, this book will fertilisation.
deal with six selected classes, namely
Chondrichthyes (cartilagenous fish, such Structure of a shark
as shark and rays), Osteichthyes (bony The body is more or less completely
fish such as tilapia and tuna), Amphibia covered by placoid scales and has a
(amphibians, such as salamanders, frog lateral line that runs along the side of its
and toad), Reptilia (reptiles, such as lizard, body for detecting water movement and
crocodile, and snake), Aves (birds, such vibrations. Its pectoral fin is anterior to the
as hen, parrot, eagle, and pigeon), and pelvic fins; usually the latter has claspers
Mammalia (mammals, such as human, (pterygopodes), which are used as
mouse, bat, rabbit and monkey). copulatory organs in males, and the caudal
fin is heterocercal in shape. The mouth
Class Chondrichthyes is located ventrally with teeth, which
All members of this class have paired are constantly replaced (polyptodont
fins, paired nares, jaws, and a two dentition). It has small eyes, which have
chambered heart. Additionally, they lower lids used to cover the eyes during
have placoid scales and endoskeletons feeding (Figure 3.60). The gill slits are
made up of cartilage. Members of class naked (no operculum) and has a spiracle,
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